Electric power steering device

ABSTRACT

An electric power steering device enabled to surely detect a fault of its motor current detector circuit by avoiding the disturbance by an electrically insulated oxide film formed on the contact surface between the commutator and the brush of the motor. An ignition key is turned on, then the motor applied voltage is increased with time to break the oxide film so that the motor current flows normally. The estimated motor current is then compared with the detected motor current. When the absolute value of the difference between those estimated motor current and detected motor currents is over a predetermined limit value, it is determined that the motor current detection circuit is defective. It is also possible to break the oxide film by integrating each difference between the motor current command value and the detected motor current value, thereby increasing the current control value step by step and increase voltage applied to the motor.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an electric power steeringdevice for motor vehicles. More particularly, the present inventionrelates to an electric power steering device that can detect faults tooccur in the motor current detecting means.

[0003] 2. Prior Art

[0004] An electric power steering device employed for a motor vehicledetects a steering torque generated at a steering shaft by an operationof the steering wheel and a speed of the motor vehicle and drives themotor according to detected signals, thereby assisting the steeringpower of the steering wheel. An electronic control circuit is used tocontrol such an electric power steering device as follows: a value of acurrent to be supplied to the motor is calculated based on the steeringtorque detected by a torque sensor and the vehicle speed detected by avehicle speed sensor and the supply current is controlled based on thecalculation result.

[0005] Concretely, the electronic control circuit controls the supplycurrent so that a large assist steering power is supplied to thesteering wheel when the steering torque is generated by an operation ofthe steering wheel and the detected vehicle speed is zero or low, and asmall assist steering power is supplied to the steering wheel when thespeed of the motor vehicle is high, thereby optimizing the supply of theassist steering power in accordance with the running state of the motorvehicle.

[0006] In such an electric power steering device, the actual currentthat flows in the motor is fed back and controlled so that the currentmatches with the target value calculated based on the steering torqueand the vehicle speed. The electric power steering device is thusprovided with a motor current detecting means for detecting the currentthat flows in the motor.

[0007] In such an electric power steering device, if the motor currentdetecting means breaks down, accurate motor current measurement isdisabled and accordingly, an excessive current flows in the motor. As aresult, an excessive assist steering power is supplied to the steeringwheel or a sufficient current is not supplied to the motor. The assiststeering power to be supplied to the steering wheel will thus becomeinsufficient.

[0008] Furthermore, an operation check is usually done for thecontrolling device of the motor vehicle at the engine start-up time. Anoperation check is also done for the motor current detecting means atthis time. And, when a current is supplied to the motor in the operationcheck, the motor rotates. If the motor shaft is coupled with thesteering mechanism at this time, the steering wheel also rotates,thereby an unexpected accident might occur.

[0009] To avoid such an accident, Japanese Patent Laid Open PublicationNo. H8-91239 (91239/1996) proposes the use of a fault determining means.According to the invention, a fault to occur in the motor currentdetecting means is determined based on a current value expected when avoltage is applied to the motor only for a short time assumed to belarger than the electrical time constant and smaller than the mechanicaltime constant of the motor, and a motor current detected by the motorcurrent detecting means itself.

[0010] The fault determining means of the above-described motor currentdetecting means determines a fault based on a voltage applied to themotor only for a short time just after the engine is started up byturning on the ignition key, that is, only for a time whose value islarger enough than the electrical time constant and smaller enough thanthe mechanical time constant of the motor. This is needed to prevent theabove described unexpected accident to be caused by an unexpectedrotation of the steering wheel when the motor begins rotating just afterthe engine starts.

[0011] The motor, when it is kept used for a certain time, causes anelectrically insulated oxide film to be formed on contact surfacesbetween the commutator and the brush of the motor. The oxide filmbecomes thicker with time, thereby the electric resistance between thecontact surfaces rises. To apply a higher voltage is thus required torotate the motor in this connection.

[0012] FIGS. 9(a) and 9(b) are diagrams showing the disturbance by suchan oxide film against motor current measurement. As to be understoodfrom FIG. 9(a), a line A denotes the normal state of the motor, in whichno oxide film is formed on the contact surface, since the motor is new.The applied voltage and current of the motor are in a proportionalrelationship with each other. The motor current increases in proportionto the rising of the applied voltage. Another line B shows a case inwhich an oxide film is formed on the contact surfaces. The motor currentdoes not increase in proportion to the rising of the applied voltage inthis case. When the applied voltage reaches the value S, however, theoxide film causes breakdown (puncture), thereby the electric resistanceof the film drops sharply. Consequently, a current corresponding to thenormal voltage comes to flow in the motor.

[0013]FIG. 9(b) shows how the applied voltage that causes breakdown ofthe oxide film rises. When the oxide film becomes thicker with time,applied voltage that causes breakdown of the oxide film will be raisedup S1, S2, S3 and S4 with time.

[0014] As described above, application of a low voltage to the motoronly for a short time might cause a problem in determination of a faultin the motor current detecting means since the motor current is notdetected or only a few motor current is detected due to the oxide filmformed on the contact surface. It might thus be determined wrongly thatthe motor current detecting means is defective.

SUMMARY OF THE INVENTION

[0015] 1. It is an object of the present invention to provide anelectric power steering device that enables sure detection of faults tooccur in the motor current detecting means free from the disturbance bythe electrically insulated oxide film to be formed on the contactsurface between the commutator and the brush of the motor for assistingthe steering torque in an operation check for the electronic controlcircuit performed just after the device engine starts up.

[0016] 2. It is another object of the present invention to provide anelectric power steering device that enables sure detection of faults tooccur in the motor current detecting means free from the disturbance bythe electrically insulated oxide film formed on the contact surfacebetween the commutator and the brush of the motor for assisting thesteering torque by increasing the voltage applied to the motor in anoperation check of the electronic control circuit performed just afterthe device engine is started.

[0017] 3. It is still another object of the present invention to providean electric power steering device that enable sure detection of faultsto occur in the motor current detecting means by increasing the voltageapplied to the motor for assisting the steering torque step by step withtime, thereby breaking the electrically insulated oxide film formed onthe contact surface between the commutator and the brush of the motor.

[0018] 4. It is still another object of the present invention to providean electric power steering device that enables sure detection of faultsto occur in the motor current detecting means by increasing the voltageapplied to the motor for assisting the steering torque step by step withtime according to a difference integrated value between a currentcommand value for the motor and the detected motor current, therebybreaking the electrically insulated oxide film formed on the contactsurface between the commutator and the brush of the motor.

[0019] 5. These and other objects of the present invention will becomemore apparent upon a reading of the following detailed descriptions anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a schematic block diagram of an electric power steeringdevice in the first embodiment of the present invention;

[0021]FIG. 2 is a block diagram of an electronic control circuit in thefirst embodiment of the present invention;

[0022]FIG. 3 is a block diagram of a motor drive circuit;

[0023] FIGS. 4(a), 4(b), 4(c) and 4(d) are diagrams showing thetransient characteristics of a motor current i and a motor angularvelocity ω, as well as a timing for sampling the motor current i;

[0024] FIGS. 5(a) and 5(b) are diagrams showing how to change a dutyratio D with time in a sampling operation;

[0025] FIGS. 6(a) and 6(b) are diagrams showing how to change the dutyratio D with time in a plurality of sampling operations;

[0026]FIG. 7 is a flowchart of the controlling operations performed bythe electronic control circuit;

[0027]FIG. 8 is a block diagram of an electronic control circuit in thesecond embodiment of the present invention; and

[0028]FIG. 9(a) and FIG. 9(b) are diagrams showing the disturbance by anoxide film formed on the contact surface between the commutator and thebrush of the motor against motor current measurement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Hereunder, the preferred embodiments of the present inventionwill be described with reference to the accompanying drawings.

[0030] [First Embodiment]

[0031] The first embodiment of the present invention will be described.FIG. 1 shows a schematic block diagram of an electric power steeringdevice in the first embodiment of the present invention. A shaft 2 of asteering wheel 1 is connected to a tie rod 8 of a wheel through areduction gear 4, universal joints 5 a and 5 b, and a pinion rackmechanism 7. The shaft 2 is equipped with a torque sensor 3 fordetecting a steering torque of the steering shaft 2. A motor 10 forassisting the steering power is connected to the shaft 2 through aclutch 9 and the reduction gear 4.

[0032] An electronic control circuit 13 for controlling the electricpower steering device receives a power from a battery 14 through anignition key 11. The electronic control circuit 13 calculates a currentcommand value according to the steering torque detected by the torquesensor 3 and the vehicle speed detected by the vehicle speed sensor 12to control the current i supplied to the motor 10 based on thecalculated current command value.

[0033] The clutch 9 is controlled by the electronic control circuit 13.The clutch 9 is engaged with the reduction gear 4 in the normal stateand disengaged from the reduction gear 4 when it is determined that theelectric power steering device is defective or when the power is turnedoff.

[0034]FIG. 2 shows a block diagram of the electronic control circuit 13.In this first embodiment, elements in the electronic control circuit 13that is mainly configured by a CPU are shown as functions to be executedby a program stored in the CPU. For example, a phase compensator 21 isnot shown as an independent hardware unit here; it is shown as afunction of phase compensation executed in the CPU. The electroniccontrol circuit 13 may not be configured by the CPU and each of theabove functions may be configured by independent hardware units(electronic circuit), of course.

[0035] Next, the functions and operations of the electronic controlcircuit 13 will be described. A steering torque signal inputted from thetorque sensor 3 is subjected to phase compensation in the phasecompensator 21 so as to improve the steering system stability. Thesignal is then inputted to a current command value calculator 22. Thevehicle speed detected by the vehicle speed sensor 12 is also inputtedto the current command value calculator 22.

[0036] The current command value calculator 22 calculates a currentcommand value I with use of a predetermined expression based on thetorque signal and the vehicle speed received respectively as describedabove. The current command value I is a target control value of thecurrent to be supplied to the motor 10.

[0037] A circuit comprising a comparator 23, a differential compensator24, a proportional calculator 25, and an integral calculator 26 is usedfor executing feedback control so as to make actual motor current valuei matche with the current command value I.

[0038] The proportional calculator 25 outputs a proportion value that isproportional to a difference between the current command value I and theactual motor current value i. The output signal of the proportionalcalculator 25 is integrated in the integral calculator 26 to improve thefeed-back system characteristics, then output as a proportion value ofthe integrated difference value.

[0039] The differential compensator 24 outputs a differentiated value ofthe current command value I to improve the response characteristics ofthe motor current value i that actually flows in the motor, with respectto the current command value I calculated by the current command valuecalculation part 22.

[0040] The differentiated value of the current command value I outputfrom the differential compensator 24, the proportion value proportionalto the difference between the current command value I and the actualcurrent value i output from the proportional calculator 25, and theintegrated value output from the integral calculator 26 are added up inthe adder 27 and the result of current control value (the duty ratio ofthe PWM signal determining a voltage to be applied to the motor) isoutput to the motor drive circuit 41 as a motor drive signal.

[0041]FIG. 3 shows a block diagram of the motor drive circuit 41. Themotor drive circuit 41 is configured mainly by a converter 44 forconverting a current control value inputted from the adder 27 to a PWMsignal and a current direction signal separately, switching elementsFET1 to FET4 (field effect transisitors), a FET gate drive circuit 45for opening/closing the gates of those switching elements. The boostingpower source 46 is used for driving the high side of each of the gatesFET1 and FET2.

[0042] The PWM signal (pulse width modulation signal) drives the gatesof the switching elements FET1 to FET2 of the H-bridge connected FETs.The PWM signal is also used to determine a duty ratio (a time ratio toturn on/off the FET gates) based on the absolute current control valuecalculated by the adder 27.

[0043] The current direction signal denotes a direction of the motorcurrent. This signal is determined by the positive/negative sign of thecorresponding current control value calculated by the adder 27.

[0044] As described above, both FET1 and FET2 are switching elements,each having a gate to be turned on/off based on the duty ratio of thePWM signal. Those FET1 and FET2 are used to control the size of themotor current. Both FET3 and FET4 are also switching elements, eachhaving a gate to be turned on/off based on the current direction signal.(When one of the FET3 and FET4 is turned on, the other is turned off.)They (FET3 and FET4) are used to switch the motor current direction,that is, the motor rotating direction.

[0045] When the FET3 is conductive, the current flows towards the motor10 in the positive direction through the FET1, the motor 10, the FET3,and the resistor R1. When the FET4 is conductive, the current flowstoward the motor 10 in the negative direction through the FET2, themotor 10, the FET4, and the resistor R2.

[0046] The motor current detection circuit 42 detects the value of thecurrent in the positive direction based on the voltage that drops atboth ends of the resistor R1 and detects the value of the current in thenegative direction based on the voltage that drops at both ends of theresistor R2. The detected actual motor current value is fed back to thecomparator 23 (see FIG. 2).

[0047] The electronic control circuit described above sets a largecurrent command value I when detected steering torque is large or thedetected vehicle speed is zero or low. When the detected steering torqueis small or the detected vehicle speed is high, the control circuit setsa small current command value I. The steering power is thus assistedoptimally according to the running state of the subject motor vehicle.

[0048] Next, an explanation will be made for how a fault is detected inthe motor current detecting means and the fail-safe processing to beperformed according to the detection result.

[0049] At first, the principles of the fault detection and the fail-safeprocessing will be described. When the ignition key 11 is turned on toapply a voltage V to the motor, a relationship in the followingexpression (1) is established between the voltage V that flows betweenmotor terminals and the current i that flows in the motor.

V=L di/dt+Ri+k _(T)ω  (1)

[0050] Here, the k_(T) denotes a counter electromotive force constantand the ω denotes a motor angular speed, the L denotes a motorinductance, and the R denotes an inter-terminal resistance of the motor.

[0051] The mechanical time constant Tm of the motor is obtained bydividing the inertia moment J of the motor by the viscosity resistance Bof the motor and represented as Tm=J/B. The electrical time constant Teof the motor is obtained by dividing the inductance L of the motor bythe resistance R of the motor and represented as Te=L/R.

[0052]FIG. 4 shows the transient characteristics of the motor current iand the motor angular velocity ω, as well as a timing for sampling themotor current when the time T is set smaller enough than the mechanicaltime constant Tm of the motor and larger enough then the electrical timeconstant Te of the motor (Te<<T<<Tm) and a voltage V is applied to themotor just for a time between the initial state and the time T.

[0053]FIG. 4(a) shows a relationship between the voltage V applied tothe motor and the application time. A certain voltage V is applied tothe motor until the time T0 before the motor current sampling begins.When the sampling begins, the duty ratio changes, thereby the voltage Vapplied to the motor changes with time.

[0054]FIG. 4(b) shows a relationship between the motor current and theapplication time. As shown in FIG. 4(b), when the voltage V is appliedto the motor, the motor current i rises quickly (the electrical timeconstant Te<<applied time T of the voltage V) and the constant current iflows in the motor. The “is” denotes an estimated motor current value(to be described later).

[0055]FIG. 4(c) shows a relationship between the angular velocity ω ofthe motor and the application time. As shown in FIG. 4(c), themechanical time constant Tm of the motor is large and the angularvelocity ω of the motor is almost zero, that is, the motor does notrotate for the time T in which the voltage V is applied to the motor. Inthis connection, when the voltage V to be applied to the motor isdetermined so that the estimated motor current “is” is set lower thanthe value corresponding to the static friction torque of the steeringmechanism, the condition that the motor does not rotate unexpectedly issatisfied.

[0056]FIG. 4(d) shows a timing for sampling the motor current. As shownin FIG. 4(d), the sampling begins at the time T0 after the voltage V isapplied to the motor.

[0057] According to the transient characteristics of the motor current iand the motor angular velocity ω, the motor current rises when the timeT0 is up, then the voltage V is applied to the motor. The time T0 is alittle earlier than the time T. And, because the motor hardly rotateswhile the constant current i flows in the motor, the angular velocity ωand the differentiated value of the motor current i becomesapproximately zero.

[0058] Consequently, the above expression (1) can be replaced with thefollowing expression (2).

V=Ri  (2)

[0059] The estimated motor current “is” is thus obtained by dividing thevoltage V between motor terminals by the internal resistance R andrepresented by the following expression (3).

is=V/R  (3)

[0060] As to be understood from the expression (3), the estimated motorcurrent “is” does not include any of the counter-electromotive forcek_(T)ω and the regenerative voltage L·di/dt item of the motor, so thatthe motor current “is” can be estimated free from the disturbance by thecounter electromotive force and the regenerative voltage of the motor.

[0061] The voltage applied to the motor may be detected directly fromthe voltage V between motor terminals or as follows.

[0062] The voltage V between motor terminals is related to the currentcontrol value (the duty ratio of the PWM signal) supplied to the motoras shown in the following expression (4).

V=V _(BAT) ·D  (4)

[0063] Here, the V_(BAT) denotes a battery voltage and the D denotes aduty ratio of the PWM signal.

[0064] Consequently, the expression (3) for representing the estimatedmotor current “is” can be replaced with the following expression (5).

is=(V _(BAT) ·D)/R  (5)

[0065] Hereinafter, a description will be made for both configurationand operation of the fault determination of the motor current detectingmeans and the fail-safe processing with reference to FIG. 2.

[0066] When the ignition key 11 is turned on, a voltage is applied tothe motor only for a predetermined time T preset in a timer TM (notshown). The on-state of the ignition key 11 is detected by the ignitionkey ON detector 31 and the detected signal is inputted to the faultdetector 32. The fault detector 32 also receives a battery voltageV_(BAT) detected by the battery voltage detector 36 and a currentcontrol value (duty ratio D of the PWM signal), which is an input signalof the motor drive circuit.

[0067] Furthermore, sampling of the motor current i begins at thepredetermined time T0 (T0<T) preset in the timer TM (not shown) and themotor current value i detected by the motor current detection circuit 42is inputted to the fault detector 32. The sampling is performed just forthe predetermined time T_(s) preset in the timer TM.

[0068] The fault detector 32 calculates an estimated current value “is”by substituting the battery voltage value V_(BAT), the duty ratio D ofPWM signal, and the resistance R between motor terminals for theexpression (5), then compares the result of calculated current value“is” with the motor current value “i” detected by the motor currentdetection circuit 42 as a sampling value. As a result, when thedifference of absolute value |is−i| is larger than a predeterminedallowable value Δi, it is determined that the motor current detectioncircuit 42 is defective.

[0069] When it is determined that the motor current detection circuit 42is defective, the fail-safe processor 33 is actuated to turn off a failrelay 34 and open a contact 34 a so that power supply to the motor 10 isshut off and the electric power steering device is set in no-operationcondition.

[0070] The fault determination for the motor current detection circuit42 might be taken as a real fault or wrong fault. The wrong fault iscaused by that the contact surface between the commutator and the brushof the motor is covered by an oxide film, thereby no motor current isdetected or only a slight motor current is detected.

[0071] In order to prevent such a wrong fault determination, a voltagebetween motor terminals, that is, a voltage applied to the motor isincreased step by step with time to break the oxide film, therebyeliminating the disturbance by the oxide film against the motor currentdetection. After this, the motor current i is detected to determinewhether or not the motor current detection circuit 42 is defective.Hereinafter, the configuration of the electric power steering devicerequired for this processing will be described.

[0072] As shown in the expression (4), the voltage between motorterminals, that is, the voltage V applied to the motor is determined bythe duty ratio D of PWM signal and the battery voltage value V_(BAT).Thus, the voltage between motor terminals, that is, the voltage Vapplied to the motor can be changed by changing the duty ratio D.

[0073] Here, a description will be made for two methods for changing theduty ratio D of PWM signal to change the motor applied voltage V: onemethod changes the duty ratio D with time in one sampling operation andthe other method changes the duty ratio D based on the number of timefor sampling done by a plurality of sampling operation.

[0074]FIG. 5(a) and FIG. 5(b) show diagrams for describing a method forchanging the duty ratio D with time during the one sampling operation.As shown in FIG. 5(a), the duty ratio D is changed from D1 to D2 in thesampling operation between the times T1 and T2 as shown in line A. Thevoltage V applied to the motor at this time is changed in proportion tothe increase of the duty ratio D as shown in FIG. 5(b) as follows; theon-time of the voltage V applied to the motor becomes longer graduallyas shown in line C, and accordingly the average value of the voltage Vincreases from V1 to V2 gradually as shown in line B.

[0075]FIG. 6(a) and FIG. 6(b) show diagrams for describing a method forchanging the duty ratio D with time in a plurality of samplingoperations. As shown in FIG. 6(a), the duty ratio, which is D1 in thefirst sampling operation, rises more and more in the subsequent samplingoperation. In the (n)-th sampling operation, the duty ratio D becomesD2. The duty ratio is assumed to be fixed in each one sampling operationin this case as shown in FIG. 6(b), line B. The voltage V applied to themotor at this time extends its on-time longer and longer in thesubsequent sampling operation. As a result, the average value of thevoltage V applied to the motor changes gradually from V1 to V2 as shownin FIG. 6(b), line C.

[0076] Duty ratio D1 is the minimum duty ratio, which corresponds to theminimum necessary voltage V for breaking an oxide film as describedabove. Duty ratio D2 is the maximum duty ratio, which corresponds to themaximum voltage V applied to the motor just before the motor rotates,thereby the steering wheel begins rotating.

[0077] Any of the above methods can be selected for changing the dutyratio D to increase the voltage V applied to the motor.

[0078]FIG. 7 shows a flowchart for controlling the operation of thefault detector 32 when the method for changing the duty ratio D based onthe number of times of sampling done by a plurality of samplingoperations.

[0079] At first, the fault detector 32 is initialized, and the timer TMis started (step P1). Then, the battery voltage value V_(BAT) and thePWM signal duty ratio D are read sequentially (steps P2 and P3). It isassumed here that D1 (the minimum duty ratio) is set as the initial dutyratio D, after that it is updated when duty ratio is changed.

[0080] The voltage V corresponding to the set duty ratio D is applied tobetween terminals of the motor (step P4). When the predetermined time T0preset in the timer TM is up (step P5), the motor current sampled valuei is read from the motor current detection circuit 42 (step P6). Theestimated motor current “is” is calculated by the expression (5) (stepP7) to determine whether or not the absolute value |is−i| is larger thanthe predetermined allowable value Δi (step P8). When the result in stepP8 is NO (not larger), it is determined that no fault has occurred,thereby control goes to the normal processing.

[0081] When the result in step P8 is YES (larger), the duty ratio D ischanged to break the oxide film, since the motor current detector mightbe defective and/or an oxide film is formed on the contact surfacebetween the commutator and the brush of the motor, thereby the motorcurrent might not be detected accurately. It is determined whether ornot the set duty ratio is D2 (maximum value) (step P9). When the resultis YES (D2), it is estimated that the oxide film is already broken.Consequently, it is determined that the motor current detection circuit42 is defective and the fail-safe processing is performed while the dutyratio D2 is kept as is (step P10), then the processing is terminated.

[0082] When the result in step P9 is NO (not D2), the battery voltageV_(BAT) is read, then the duty ratio D is increased by one step (stepsP11 and P12). After that, program controlling is jumped to step P4.

[0083] A voltage value corresponding to the mechanical time constant Tmof the motor is assumed as the upper limit for the voltage V applied tothe motor for preventing the motor rotation in the fault detectionprocessing.

[0084] As described above, in the first embodiment, the motor currentdetecting circuit is checked for faults just after the ignition key isturned on. The fault is detected by comparing the estimated motorcurrent value based on the motor current command value with the actualmotor current value detected by the motor current detection circuitwhile the motor current command value is set only for a time T whosevalue is smaller enough than the mechanical time constant Tm and largerenough than the electrical time constant Te of the motor (Te<<T<<Tm) andthe current control value is changed with time. The method thus makes itpossible to determine faults of the motor current detecting circuitwhile the motor does not rotate.

[0085] Furthermore, the motor current detection circuit is also checkedfor faults while a current flows in the motor only for a short time.However, the fault that is detected at this time might not be a realone. This is because a similar fault is often detected when an oxidefilm is formed on the contact surface between the commutator and thebrush of the motor, thereby the motor current cannot be detectedaccurately. In this first embodiment, to avoid such a problem, the dutyratio D that determines the motor voltage is changed with time toincrease the voltage applied to the motor step by step so that the oxidefilm on the contact surface is broken to enable correct detection of themotor current. Consequently, faults of the motor current detectioncircuit come to be always detected accurately.

[0086] Furthermore, in the first embodiment, faults of the motor currentdetection circuit can be detected just after the ignition key is turnedon while the motor angular velocity ω is almost zero and accordingly,the motor does not rotate. Then an accident that the steering wheelhappens to rotate while the motor current detection circuit is checkedfor faults can be prevented.

[0087] [Second Embodiment]

[0088] Next, the second embodiment of the present invention will bedescribed.

[0089]FIG. 8 shows a block diagram of an electronic control circuit 13.In this second embodiment, the same reference numerals will be used forthe same elements as those in the first embodiment, avoiding redundantdescription. In the second embodiment, elements of the electroniccontrol circuit 13 that is mainly configured by a CPU are shown asfunctions to be executed by a program stored in the CPU. For example, aphase compensator 21 does not denote an independent hardware unit here;it is shown as a function of phase compensation to be executed in theCPU. The electronic control circuit 13 may not be configured by the CPUand each of the above functions may be configured by independenthardware units (electronic circuit), of course.

[0090] Hereunder, the functions and operations of the electronic controlcircuit 13 will be described. A steering torque signal inputted to atorque sensor 3 is subjected to phase compensation by a phasecompensator 21 to improve the stability of the steering system, theninputted to a current command value calculator 22. A vehicle speeddetected by a vehicle speed sensor 12 is also inputted to the currentcommand value calculator 22. The current command value calculator 22calculates a current command value I with use of a predeterminedexpression based on a torque signal and a vehicle speed signal inputtedas described above. The current command value I is a target controlvalue of the current to be supplied to a motor 10.

[0091] A current deviation calculator/proportional integrator 53 is acalculating element that calculates a difference Δi between the currentcommand value I output from the current command value calculator 22 andthe actual motor current value i detected by the motor current detectioncircuit 42, and perform a proportional integration (PI operation)according to the difference Δi, thereby outputting a current controlvalue E for controlling the motor 10.

[0092] Next, the operation of the current deviationcalculator/proportional integrator 53 will be described. While the motorcurrent value i is detected normally, the difference value Δi becomesapproximately zero. The current control value E output from the currentdeviation calculator/proportional integrator 53 thus becomesapproximately equal to the current command value I, and the motor 10 isdriven by the fed-back control so that the difference value Δi becomeszero.

[0093] While the motor current value i is not detected normally, thedifference value Δi between the current command value I and the detectedactual motor current value i is large. Consequently, the current controlvalue E increases step by step as a result of the proportionalintegration (PI operation) performed by the difference value Δi betweenthe current command value I and the detected actual motor current valuei. Thus, the voltage V applied to the motor 10 between terminals risesstep by step.

[0094] When the contact surface between the commutator and the brush ofthe motor 10 is covered by an oxide film, the initial motor currentvalue i is detected only slightly. Then, the slightly detected currentvalue i is fed back to the current deviation calculator/proportionalintegrator 53. As the difference values Δi are integrated, the currentcontrol value E increases step by step, thereby the motor appliedvoltage V rises step by step.

[0095] When the motor applied voltage V exceeds a certain value, theoxide film on the contact surface is broken. A large current thus comesto flow in the motor suddenly in correspondence to the high voltage Vapplied according to the increased current control value E. After this,however, the motor current value i comes to be detected normally,thereby the motor is driven by the fed back control so that thedifference value Δi between the current command value I and the detectedactual motor current value i becomes zero.

[0096] The motor current checker 54 determines whether or not the motorcurrent value i detected by the motor current detection circuit 42 iswithin the preset limit value. When the result is NO (not within thelimit value), the motor current checker 54 outputs a fault signal. Thesignal denotes that a fault is detected in the motor current detectioncircuit 42. When the result is YES (within the limit value), the motorcurrent checker 54 outputs a no-fault signal.

[0097] The fault detector 55 determines whether the motor currentdetection circuit 42 is faults or not and output a fault determinationsignal based on a plurality of signals output from the motor currentchecker 54, a signal output from the ignition key ON detector 31 thatdetects the on-state of the ignition key, and a signal output from thebattery voltage detector 36 that determines whether battery voltage isnormal or not.

[0098] The fail-safe processor 56 actuates a relay circuit 34 accordingto the fault determination signal output from the fault detector 55 toshut off the power supply to the motor 10. The fault determination andthe fail-safe processing will be described in detail later.

[0099] The configuration of the motor drive circuit 41 is the same asthat in the first embodiment. The description will thus be omitted here.

[0100] The electronic control circuit described above can assist thesteering power optimally according to the running state of the subjectmotor vehicle, since a large current command value I is set when thedetected steering torque is large and the detected vehicle speed is zeroor low and a small current command value I is set when the detectedsteering torque is small and the detected vehicle speed is high.

[0101] Next, a description will be made for how a fault in the motorcurrent detection circuit 42 will be determined and how a fail-safeprocessing is performed based on the detection result.

[0102] At first, the principles of the fault determination and thefail-safe processing will be described. When the ignition key is turnedon to apply a voltage V to the motor 10, a relationship as shown in thefollowing expression (1) is established between the voltage V betweenmotor terminals and the motor current i.

V=L·di/dt+Ri+k _(T)ω  (1)

[0103] Here, the k_(T) denotes a counter-electromotive force constant ofthe motor and the ω denotes an angular velocity of the motor. The Ldenotes an inductance of the motor and the R denotes a resistancebetween motor terminals.

[0104] The mechanical time constant Tm of the motor is obtained bydividing the inertia moment J of the motor by the viscosity resistance Bof the motor and represented as Tm=J/B. The electrical time constant Teof the motor is obtained by dividing the inductance L of the motor bythe resistance R of the motor and represented as Te=L/R.

[0105] Next, a description will be made for the transientcharacteristics of the motor current i and the motor angular velocity ω,as well as a timing for sampling the motor current when the time T isset smaller enough than the mechanical time constant Tm of the motor andlarger enough then the electrical time constant Te of the motor(Te<<T<<Tm) and a voltage V is applied to the motor just for a timebetween the initial state and the time T with reference to FIG. 4(a) andFIG. 4(b) that are also referred to in the first embodiment.

[0106]FIG. 4(a) shows a relationship between the voltage V applied tothe motor and the application time T. A certain voltage V0 is applied tothe motor until the time T0 before the motor current sampling begins.After the sampling begins, the voltage V increases with time step bystep.

[0107]FIG. 4(b) shows a relationship between a motor current and acurrent application time. In the normal state, that is, when no oxidefilm is formed on the contact surface between the commutator and thebrush of the motor, the motor current rises quickly (electrical timeconstant Te of the motor<<applying time T of the voltage V) in responseto the voltage V applied to the motor, thereby a constant current iflows in the motor.

[0108]FIG. 4(c) shows a relationship between an angular velocity ω ofthe motor and an application time. As shown in FIG. 4(c), the mechanicaltime constant Tm of the motor is large and the angular velocity of themotor is almost zero, that is, the motor does not rotate for the time Tin which the voltage V is applied to the motor. In this connection, whenthe voltage to be applied to the motor is determined so that theestimated motor current “is” is set lower than the value correspondingto the static friction torque of the steering mechanism, the conditionthat the motor does not rotate unexpectedly is satisfied.

[0109]FIG. 4(d) shows a timing for sampling the motor current i. Asshown in FIG. 4(d), the sampling begins at T0 after the voltage V isapplied to the motor. The Ts denotes a sampling time.

[0110] As described above, the motor current detection circuit 42 faultdetermination is done by sampling the motor current value i by aplurality of times just after the ignition key is turned on.

[0111] Hereinafter, a description will be made for how a fault in themotor current detection circuit 42 of the present invention isdetermined, as well as for the configuration and operation of thefail-safe processing based on the fault determination with reference toFIG. 8.

[0112] The motor current detection circuit 42 might fail in thedetection of the motor current value i in the following two cases. Inone case, the motor current does not flow or flows only slightly due tothe disturbance by the oxide film formed on the contact surface betweenthe commutator and the brush of the motor while the motor currentdetection circuit 42 is normal in operation. In the other case, themotor current detection circuit 42 itself is defective.

[0113] To avoid such wrong fault detection, the present inventionenables the motor current value i to be detected after the oxide film onthe contact surface is broken, thereby the fault of the motor currentdetection circuit 42 itself is detected accurately.

[0114] When the ignition key 11 is turned on, the voltage V is appliedto the motor only for a predetermined time T preset in a timer TM (notshown). The on-state of the ignition key 11 is detected by the ignitionkey ON detector 31 and the detected signal is inputted to the faultdetector 55. The fault detector 55 also receives a battery voltageV_(BAT) detected by the battery voltage detector 36.

[0115] At the time of the detection of fault of the motor currentdetection circuit 42, no steering torque is generated and the vehiclespeed is zero. Therefore, the motor is not driven to generate an assisttorque. Consequently, a predetermined current command value I for faultdetection is output from the current command value calculator 22 so asto apply the voltage V to the motor 10.

[0116] On the other hand, sampling of the motor current value i isstarted after predetermined time T0 (T0<T) preset in the timer TM (notshown) is up. The sampling continues only for the predetermined time Tspreset in the timer TM, as shown in FIG. 4(d).

[0117] As described above, the current deviation calculator/proportionalintegrator 53 calculates the difference Δi between the current commandvalue I and the detected actual motor current value i and performs aproportional integration (PI operation) according to the difference Δi,thereby outputting a current control value E for controlling the motor10. When the contact surface between the commutator and the brush of themotor 10 is covered by an oxide film, the difference value Δi is largeand the current command value I increases, thereby the current controlvalue E increases step by step. Consequently, the motor applied voltageV also rises step by step. When the motor applied voltage V exceeds acertain value, the oxide film is broken, thereby the normal currentflows in the motor. The motor current i is thus detected.

[0118] The above processings are always performed in the currentdeviation calculator/proportional integrator 53 regardless of whether anoxide film is formed on the contact surface or not, the motor currentdetection circuit 42 can detect the motor current value i free from thedisturbance by the oxide film.

[0119] In the above processings, a very large current value i isdetected at a moment when the oxide film insulation is broken due to themotor applied voltage V that increases step by step. At the nextsampling time, however, the motor current value i is detected normally.Therefore, the normal motor current value i is detected in a pluralityof sampling operations. Concretely, even when the detected motor currentis not within the limit value at a sampling time, it cannot bedetermined that the motor current detection circuit 42 is defective.Otherwise, the detection might be determined wrongly.

[0120] When the detected motor current value is not within thepredetermined limit value even in a plurality of sampling operations, itis determined that the motor current detection circuit 42 is defective,since no oxide film is formed on the contact surface in this case. It isalso possible to determine that the motor current detector 42 isdefective when the detected current value that is not within thepredetermined limit value is detected continuously in a specified numberof sampling operations.

[0121] Furthermore, because the voltage V applied to the motor risesstep by step in time series, when the detected motor currentcorresponding to the voltage V is not within the predetermined value, itmay be determined that the motor current detection circuit 42 isdefective.

[0122] The motor current checker 54 outputs a plurality of fault signalswhen the motor current value i detected by the motor current detectioncircuit 42 is not within the predetermined limit value. Each of thesignals denotes that a fault has occurred in the motor current detectioncircuit 42. When the motor current value i is within the limit value,the motor current checker 54 outputs a plurality of no-fault signals.The reason why a plurality of fault/no-fault signals are output at thistime is that the motor current value i is sampled by a plurality oftimes.

[0123] The fault detector 55 confirms that those signals are detectedthrough sampling of the motor current value i by a plurality of times inan operation check performed just after the ignition key is turned onbased on a plurality of fault or no-fault signals output from the motorcurrent checker 54, the signal output from the ignition key ON detector32, and the signal output from the battery voltage detector 36.

[0124] The fault detector 55 also determines that a fault has occurredin the motor current detecting means and outputs a fault determinationsignal to the fail-safe processor 56 when a fault signal is detectedfrom every detection result or from a detection result just afterno-fault signal is detected in time series.

[0125] The fail-safe processor 56 actuates the relay circuit 34 to openthe contact 34 a according to the received fault determination signal,then shuts off the power supply to the motor 10. The operation of theelectric power steering device is thus disabled.

[0126] The motor applied voltage V should be limited in maximum by avalue corresponding to the mechanical time constant of the motor. Thisis because otherwise an unexpected rotation of the motor might occurwhen the motor applied voltage V rises. When the motor applied voltage Vis limited by such an upper limit value corresponding to the mechanicaltime constant of the motor, an accident caused by an unexpected rotationof motor is prevented.

[0127] As described above, in the second embodiment of the presentinvention, the motor current detector circuit is checked for faults justafter the ignition key is turned on. And, a motor current command valueis set only for a time T whose value is smaller enough than themechanical time constant Tm of the motor and larger enough than theelectrical time constant Te of the motor, thereby the current deviationcalculator/proportional integrator 53 calculates a difference between acurrent command value and a detected motor current value and perform aproportional integration (PI operation) for the result based on thedifference Δi to output a current control value E for controlling themotor.

[0128] When the motor current is not detected normally, the currentcontrol value E increases with time through the above integration,thereby the motor applied voltage V rises. A high voltage is thusapplied to the motor even when an electrically insulated oxide film isformed on the contact surface between the commutator and the brush ofthe motor. The insulation of the oxide film is thus broken, thereby themotor current comes to flow normally.

[0129] Consequently, the fault detection is done free from thedisturbance by the oxide film, so that it is possible to determine thatthe motor current detector circuit is defective when the detected motorcurrent is not within a predetermined limit value.

[0130] Furthermore, it is possible to make fault detection in the motorcurrent detecting means just after the ignition key is turned on evenwhile the motor is not rotated actually. An accident that the steeringwheel comes to rotate unexpectedly can thus be avoided during faultdetection.

[0131] Although only preferred embodiments are specially illustrated anddescribed herein, it will be apparent that many modifications andvariations of the present invention are possible in light of the aboveteachings and within the preview of the appended claims withoutdeparting from the spirit and intended scope of the invention.

What is claimed is:
 1. An electric power steering device, comprising: asteering unit equipped with a motor for assisting the steering power ofitself; controlling means for controlling an output of said motoraccording to at least a signal of a steering torque generated at asteering shaft; wherein said controlling means includes: motor currentcommand value calculating means for calculating a current command valueof a current to be supplied to said motor; motor current estimatingmeans for estimating a value of said motor current based on said currentcommand value; motor current detecting means for detecting a value of acurrent that flows in said motor; fault detecting means for detecting afault in said motor current detecting means by comparing said estimatedmotor current with said detected motor current; and wherein saidcontrolling means controls so that a motor current command value is setonly for a time whose value is larger enough than an electrical timeconstant of said motor and smaller enough than a mechanical timeconstant of said motor, then a current control value is changed withtime based on said current command value to increase a voltage appliedto said motor with time and enable said fault detecting means to comparesaid estimated motor current value with said detected motor currentvalue, thereby detecting a fault in said motor current detecting means.2. The electric power steering device according to claim 1, wherein saidcontrolling means, when a fault is detected in said motor currentdetecting means, increases a voltage applied to said motor with time,starting from a voltage lower than said motor applied voltage detectedat the time of fault detection, so as to detect said fault again in saidmotor current detecting means.
 3. The electric power steering deviceaccording to claim 1, wherein said motor applied voltage is limited inmaximum by a value corresponding to said mechanical time constant ofsaid motor.
 4. The electric power steering device according to claim 1,wherein said controlling means changes the duty ratio of said voltageapplied to said motor with time and increase said voltage with time. 5.The electric power steering device according to claim 4, wherein saidcontrolling means changes the duty ratio of said voltage applied to saidmotor with time in one sampling operation and increase said voltage withtime.
 6. The electric power steering device according to claim 4,wherein said controlling means changes the duty ratio of said voltageapplied to said motor according to an increase of the number of samplingoperations and increase said voltage with time.
 7. An electric powersteering device, comprising: a steering unit equipped with a motor forassisting the steering power of itself; controlling means forcontrolling an output of said motor according to at least a signal of asteering torque generated at a steering shaft; wherein said controllingmeans includes: motor current command value calculating means forcalculating a current command value of a current to be supplied to saidmotor; motor current detecting means for detecting a current that flowsin said motor; current deviation calculating/proportionally integratingmeans for calculating each difference between a motor current commandvalue and a detected motor current and integrating calculated differenceproportionally for outputting a current control value; and faultdetermining means for determining that said motor current detectingmeans is defective when said detected motor current is not within apredetermined limit value; wherein said controlling means controls sothat said motor current command value is set only for a time whose valueis larger than said electrical time constant of said motor and smallerthan said mechanical time constant of said motor to drive said motorwith a voltage to be set based on the current control value output fromsaid current deviation calculating/proportionally integrating means soas to enable said fault determining means to determine a fault of saidmotor current detecting means.
 8. The electric power steering deviceaccording to claim 7, wherein said controlling means increases saidvoltage applied to said motor up to a value that breaks an oxide filmformed on a contact surface between a commutator and a brush of saidmotor.
 9. The electric power steering device according to claim 7,wherein said fault determining means determines a fault of said motorcurrent detecting means according to each detection result of said motorcurrent detecting means obtained in a plurality of motor currentsampling operations.
 10. The electric power steering device according toclaim 9, wherein said fault determining means does not determine thatsaid detected motor current detecting means is defective immediatelywhen said detected motor current includes values that are both withinsaid predetermined limit value and not within said predetermined limitvalue in said motor current sampling results obtained through aplurality of sampling operations, and said fault determining meansdetermines that said motor current detecting means is defective whensaid detected motor current includes a value not within saidpredetermined limit value in said sampling results obtained through aplurality of sampling operations, or said detected motor current valuesinclude a value not within said predetermined limit value in saidsampling results obtained in correspondence with said voltage applied tothe motor increased step by step in time series.
 11. The electric powersteering means according to claim 9, wherein said fault determiningmeans determines that the motor current detecting means is defectivewhen a plurality of motor currents that are not within said limit valueare detected consecutively in detection results obtained through aplurality of sampling operations performed by said motor currentdetecting means.
 12. The electric power steering device according toclaim 7, wherein said motor applied voltage is limited in maximum by avalue corresponding to said mechanical time constant of said motor.